Earth: An Introduction to Physical Geology, 10e Tarbuck & Lutgens © 2011 Pearson Education, Inc.

Geologic Time Earth, 10e - Chapter 9 Stan Hatfield Southwestern Illinois College © 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc. Relative Dating Law of superposition Developed by Nicolaus Steno in 1669 In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom. © 2011 Pearson Education, Inc.

Superposition Is Well Illustrated by the Strata in the Grand Canyon © 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc. Relative Dating Principle of original horizontality Layers of sediment are generally deposited in a horizontal position. Rock layers that are flat have not been disturbed. Principle of cross-cutting relationships Younger features cut across older features. © 2011 Pearson Education, Inc.

Cross-Cutting Relationships © 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc. Relative Dating Inclusions An inclusion is a piece of rock that is enclosed within another rock. The rock containing the inclusion is younger. Unconformity An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units. © 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc. Relative Dating Unconformity Types of unconformities Angular unconformity—tilted rocks are overlain by flat-lying rocks Disconformity—strata on either side of the unconformity are parallel Nonconformity—metamorphic or igneous rocks in contact with sedimentary strata © 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc. Formation of an Angular Unconformity © 2011 Pearson Education, Inc.

Unconformities in the Grand Canyon © 2011 Pearson Education, Inc.

Fossils—Evidence of Past Life Fossils are traces or remains of prehistoric life that are now preserved in rock. Fossils are generally found in sedimentary rock (rarely in metamorphic and never in igneous rock). Paleontology is the study of fossils. © 2011 Pearson Education, Inc.

Fossils—Evidence of Past Life Geologically fossils are important because they: Aid in interpretation of the geologic past Serve as important time indicators Allow for correlation of rocks from different places © 2011 Pearson Education, Inc.

Fossils—Evidence of Past Life Types of fossils The remains of relatively recent organisms—teeth, bones, etc. Entire animals, flesh included Given enough time, remains may be petrified (literally “turned into stone”). Molds and casts Carbonization © 2011 Pearson Education, Inc.

Fossils—Evidence of Past Life Types of fossils Others Tracks Burrows Coprolites (fossil dung) Gastroliths (polished stomach stones) © 2011 Pearson Education, Inc.

Fossils—Evidence of Past Life Conditions favoring preservation Rapid burial Possession of hard parts (skeleton, shell, etc.) © 2011 Pearson Education, Inc.

Fossils and Correlation Matching of rocks of similar ages in different regions is known as correlation. Correlation often relies upon fossils. William Smith (in the late 1700s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content. © 2011 Pearson Education, Inc.

Fossils and Correlation Principle of fossil succession—fossil organisms succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content. An index fossil is a geographically widespread fossil that is limited to a short span of geologic time. © 2011 Pearson Education, Inc.

Dating Rocks Using Overlapping Fossil Ranges © 2011 Pearson Education, Inc.

Dating with Radioactivity Reviewing basic atomic structure Nucleus Protons are positively-charged particles with mass. Neutrons are neutral particles with mass. Electrons are negatively-charged particles that orbit the nucleus. © 2011 Pearson Education, Inc.

Dating with Radioactivity Reviewing basic atomic structure Atomic number Element’s identifying number Equal to the number of protons Mass number Sum of the number of protons and neutrons © 2011 Pearson Education, Inc.

Dating with Radioactivity Reviewing basic atomic structure Isotope Variant of the same parent atom Differs in the number of neutrons Results in a different mass number than the parent atom © 2011 Pearson Education, Inc.

Dating with Radioactivity Spontaneous changes (decay) in the structure of atomic nuclei Types of radioactive decay Alpha emission Emission of two protons and two neutrons (an alpha particle) Mass number is reduced by 4, and the atomic number is lowered by 2. © 2011 Pearson Education, Inc.

Dating with Radioactivity Types of radioactive decay Beta emission An electron (beta particle) is ejected from the nucleus. Mass number remains unchanged and the atomic number increases by 1. Electron capture An electron is captured by the nucleus and combines with a proton to form a neutron. Mass number remains unchanged and the atomic number decreases by 1. © 2011 Pearson Education, Inc.

Dating with Radioactivity Parent—an unstable radioactive isotope Daughter product—the isotopes resulting from the decay of a parent Half-life—the time required for one-half of the radioactive nuclei in a sample to decay © 2011 Pearson Education, Inc.

Dating with Radioactivity Radiometric dating The percentage of radioactive atoms that decay during one half-life is always the same (50%). However, the actual number of atoms that decay continually decreases. Comparing the ratio of parent to daughter yields the age of the sample. © 2011 Pearson Education, Inc.

Radioactive-Decay Curve © 2011 Pearson Education, Inc.

Isotopes Commonly Used in Radiometric Dating © 2011 Pearson Education, Inc.

Dating with Radioactivity Radiometric dating Sources of error A closed system is required. To avoid potential problems, only fresh, unweathered rock samples should be used. © 2011 Pearson Education, Inc.

Dating with Radioactivity Dating with carbon-14 (radiocarbon dating) Half-life = 5730 years. Used to date very recent events Carbon-14 is produced in the upper atmosphere. Useful tool for anthropologists, archaeologists, and geologists who study very recent Earth history © 2011 Pearson Education, Inc.

Dating with Radioactivity Importance of radiometric dating Radiometric dating is a complex procedure that requires precise measurement. Rocks from several localities have been dated at more than 3 billion years. Confirms the idea that geologic time is immense © 2011 Pearson Education, Inc.

The Geologic Time Scale The geologic time scale is a “calendar” of Earth history. Subdivides geologic history into units Originally created using relative dates Structure of the geologic time scale An eon is the greatest expanse of time. © 2011 Pearson Education, Inc.

The Geologic Time Scale Structure of the geologic time scale Names of the eons Phanerozoic (“visible life”)—the most recent eon, which began about 540 million years ago Proterozoic Archean Hadean—the oldest eon © 2011 Pearson Education, Inc.

The Geologic Time Scale Structure of the geologic time scale Era—subdivision of an eon Eras of the Phanerozoic eon Cenozoic (“recent life”) Mesozoic (“middle life”) Paleozoic (“ancient life”) Eras are subdivided into periods. Periods are subdivided into epochs. © 2011 Pearson Education, Inc.

The Geologic Time Scale © 2011 Pearson Education, Inc.

The Geologic Time Scale Precambrian time Nearly 4 billion years prior to the Cambrian period Not divided into smaller time units because the events of Precambrian history are not known in great enough detail First abundant fossil evidence does not appear until the beginning of the Cambrian period. © 2011 Pearson Education, Inc.

The Geologic Time Scale Difficulties in dating the geologic time scale Not all rocks can be dated by radiometric methods. Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed. The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed. © 2011 Pearson Education, Inc.

The Geologic Time Scale Difficulties in dating the geologic time scale Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth’s history and to arrive at ages. © 2011 Pearson Education, Inc.

Dating Sedimentary Rocks © 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc. End of Chapter 9 © 2011 Pearson Education, Inc.